Integrated circuits are becoming increasing difficult to construct because the feature sizes at minimum dimensions for the various structures which make up the circuits. Modern integrated circuits, or chips, enjoy widespread use in a great variety of devices and systems, so the circuits must be efficiently and cost effectively constructed. Chips are typically manufactured using highly complex techniques based on elaborate, multi-step processes including some type of lithographic printing and chemical processing. The processing is performed on a semiconductor substrate such as silicon, although other materials, including compound semiconductors formed from materials such as gallium and arsenic, are also used. Modern fabrication processes may consist of hundreds or thousands of manufacturing and handling steps. In order for a given electronic device or system to function as designed, each of the steps in the fabrication process must be completed successfully. Each lithographic, process, and handling step has a unique purpose. For example, the lithographic steps are used to expose, or “print,” desired patterns and features onto a semiconductor substrate. The printing process is reminiscent of photographic processes that involved shining light through a negative to expose light sensitive paper. Similarly, specific wavelengths of light are shone through masks to print extremely fine structures onto a substrate. Following a printing step, the physical structures are revealed when chemical processing is applied to the substrates. Returning to the photography analogy, the chemical processing “develops” the fine structures and patterns on the substrates much like certain chemicals develop images on photographic paper during traditional photographic processing. The chemical processing removes, at a particular level on the substrates unwanted superfluous material while leaving intact the desired structures and patterns printed during the lithographic step. The lithographic and chemical processing steps are repeated numerous times to produce the desired electronic devices and systems.
The printing of minimum-size features and structures onto a substrate has been performed using light-based approaches. Light shines through a mask to transfer the pattern of the mask onto the substrate. The printing may be performed using positive (direct copy) or negative (reversed copy) methods. Since the minimum feature sizes of patterns and structures on the chips are now comparable to or smaller than the wavelength of visible light, lithographic techniques have been enhanced. Improved mask techniques have been developed, which, along with the development of light sources with shorter wavelengths, have helped support printing of ever-smaller features and structures. The combination of improved masks and decreased light wavelengths has significantly improved the resolution of fine features and structure sizes. After the features and structures have been printed, advanced chemical processing must resolve the small design details in such a way that all of the resulting features and structures function as designed. In order to produce a working chip, the lithographic and chemical processing steps must all be capable of functioning together to accurately produce the desired patterns and structures.